Sonntag, 12. Mai 2013

Petals for Harmattan - A pure Qt4/Qt5 JS/QML puzzle game

Next up in my list of things I did in the last weeks/months and never blogged about is Petals (Nokia Store link), a "beautiful, brain-teasing puzzle game for 1-4 players" if the game's website is to be believed (I would like to think it is...). As always, there's some technical details about the porting and creation of this game. While another recent game (Tetrepetete) has been done on a low level (C++ using no frameworks, and interfacing with multiple front-ends directly, including an OpenGL ES frontend, a console-based ncurses frontend(!) as well as a server-sent events/XHR/Canvas2D-based HTML5 frontend(!!)), this one is approaching things from a very high level: JavaScript.

The gameplay logic of the game is implemented in pure JavaScript (without any QML dependencies), so it could easily be ported to, say, HTML5, but for integration reasons, QML seemed like the better choice for a release on the N9/Harmattan. Also, writing things in JavaScript wouldn't preclude a console-based frontend using nodejs and node-ncurses from happening should the need arise (making the flowers look good in ASCII art would be the challenge there - or cheating by using libcaca). Ok, ok - stop cursing, I'll stop talking about curses (cue laugh track).

Writing pure QML applications has the advantage of easing porting to Qt 5. While QtQuick 1.1 still exists on Qt 5 (and is the only QML option at the moment if you are also targetting iOS), QtQuick 2.0 is usually the better choice for performance reasons.

In my case, the changes necessary to port from QtQuick 1.1 to QtQuick 2.0 were:

Instead of assigning a JavaScript function to a property to create a dynamic property binding (item.someprop = function() { return otheritem.otherprop * 3.0; }), this function has to be wrapped in a call to Qt.binding() in Qt 5 (see "Creating Property Bindings from JavaScript" in the Qt 5 docs)

And that's basically it. Of course, as this is a full-screen game with custom UI, no platform-specific components (such as Harmattan Components or Sailfish Silica) are used, so porting is a bit easier there (no need to "wait" for specific components to be compatible with QtQuick 2.0, which might realistically not happen at all for Harmattan Components). More screenshots of Petals and download links for multiple platforms can be found on the Petals Website.